Furfural is a useful precursor for industrial chemicals, in particular to produce furan and its derivatives.
Furfural may be produced from the hydrolysis of feedstock comprising lignocellulosic biomass. Lignocellulosic biomass comprises mainly hemicelluloses and cellulose, and smaller portions of lignin and protein. Hemicelluloses are a branched polysaccharide of heterogeneous monosaccharide content. Their molecular structure includes the five-carbon monosaccharides (‘pentose(s)’) xylose and arabinose, as well as the six-carbon monosaccharides (‘hexose(s)’) mannose, galactose and rhamnose. Due to their xylose and arabinose content, hemicelluloses are a suitable source of monomeric and polymeric pentoses. In comparison, cellulose is a linear-polysaccharide made up of polymerised glucose (a six-carbon monosaccharide/hexose). Compared to cellulose, hemicelluloses are easier to breakdown into their constituent monosaccharides.
Commercially available feedstock comprising lignocellulosic biomass includes bagasse, which is the fibrous matter that remains after sugarcane or sorghum stalks are crushed their juices extracted. An established continuous process for the production of furfural from bagasse is the Rosenlew process, the details of which are discussed in “The Chemistry and Technology of Furfural and its Many By-Products”, 1st Edition, K. Zeitsch, pages 48-51 and 303-306.
WO2012041990 describes the production of furfural from bagasse-derived hemicellulose, via its gaseous acid catalysed hydrolysis to pentoses, which are then dehydrated to produce furfural.
WO2016025678 describes the production of furfural, where initially hemicellulose is hydrolysed in a solution comprising α-hydroxysulfonic acid, a portion of the α-hydroxysulfonic acid is then removed from the hydrolysis reaction product to produce an acid-removed stream, and finally the acid-removed stream is subjected to a dehydrating step to produce furfural.
WO2016025679 describes a hydrolysis step, which is buffered to, preferably, less than pH 1, followed by a dehydrating step to produce furfural.
In both WO2016025678 and WO2016025679, during the dehydration reaction step, a “bi-phasic” dehydration reaction mixture is formed by the addition of ‘a water-immiscible organic phase’ (i.e. a solvent) into the dehydration reaction mixture. The dehydration reaction mixture is then separated into an aqueous product stream, and an organic product stream comprising a portion of furfural. However, WO2016025678 and WO2016025679 do not disclose how furfural can be fully recovered and purified from the organic product stream comprising furfural. Further, WO2016025678 and WO2016025679 do not disclose how furfural remaining in the aqueous product stream can be efficiently recovered and purified from the aqueous product stream.
Solvent extraction of furfural from an aqueous environment is complicated by the carry-over of water into the organic phase, as well as the formation of a furfural-water azeotrope. The extent of the water carry-over depends on the solvent used. Oxygenate solvents, such as those of phenolic compounds, carry more water into the organic phase (approximately around 10,000 ppm to around 40,000 ppm), as compared to aromatic solvents (approximately around 200 ppm to around 1,000 ppm). Further, if furfural is present in an aqueous environment, a furfural-water azeotrope can be formed. It is known in the art of extracting chemical compounds from mixtures of compounds that the presence of any azeotrope increases the energy consumption of a given process, as well as complicating the step and the equipment needed for that process.
Aromatic solvents have a lesser tendency to carry-over water and therefore are less likely to favour the formation of a furfural-water azeotrope, so on the face of it, aromatic solvents seem good candidates for the extraction of furfural. However due to furfural's properties, aromatic solvents' ability to extract furfural is lower than that of oxygenate solvents, which potentially decreases the overall furfural recovery when aromatic solvents are used.
Processes for the production of furfural from biomass lead to the formation of humins and tar, which can adversely interfere with the extraction and purification of furfural. Humins are dark, amorphous and undesirable acid by-products and resinous material resulting from sugars, and other organic compound degradation. Tar is a generic reference to organic material which is insoluble in water, which is dark in colour, and which tends to become viscous and very dark to almost black when concentrated. Particularly, the separation of an organic phase from an aqueous phase, and/or the later separation or purification steps can be adversely affected.
The inventors of the present invention have observed that such problems due to the formation of humins and tar are applicable in the formation, and during the extraction and purification of furfural from lignocellulosic biomass, but may be alleviated by the use of oxygenate solvents, rather than aromatic solvents.
Regarding energy consumption, the Rosenlew process uses azeotropic distillation to isolate furfural from the reaction mix by, and does not use solvent extraction. The Rosenlew process consumes about 10 tonnes of steam to recover each tonne of furfural.
It would, therefore, be advantageous to provide a process for the recovery of furfural that is more energy-efficient, which provides a high-yield of furfural than the prior art processes, as well as one which does not suffer from the interference of humins and tar.